There has recently been a growing demand for environmentally friendly hybrid electric vehicles (HEVs), and to improve the efficiency of HEV power systems, there is a need for the use of electrochemical capacitors that are capable of supplying a large amount of current within a much shorter time than the existing battery systems for energy storage.
Such electrochemical capacitors can be broadly classified into two types: an electrical double-layer capacitor having an electrical double layer formed between a carbon-based electrode and an electrolyte; and a supercapacitor that exploits the pseudocapacitance generated by the reversible faradaic surface redox reaction at the electrode/electrolyte interface, and stores the generated charges.
Electrodes for such supercapacitors are mainly made using conductive polymers (e.g., polyaniline and polypyrrole) or metal oxides. A supercapacitor having an electrode made of a conductive polymer may have a specific capacitance of up to 800 F/g, but it is disadvantageous in terms of long-term life stability and cycle characteristics, which limits the commercialization of such a supercapacitor. In contrast, transition metal oxide-based supercapacitors have attracted increasingly more attention due to their high specific capacitance, long operation time, and high output. In particular, numerous studies have focused on the application of a RuO2 electrode having high electrical conductivity and specific capacitance. However, the cost of expensive RuO2 impedes the fabrication of such an electrode for a supercapacitor on a commercial scale despite its excellent supercapacitive properties.
Electrode materials such as MnO2, NiOx, CoOx, V2O5, and MoO3 have also been studied as potential replacements for RuO2, particularly, environmentally friendly and inexpensive manganese oxide (MnO2). In order to use manganese oxide having a lower electrical conductivity than RuO2 for the manufacture of a supercapacitor with high-rate characteristics, it is important to increase the electrical conductivity of the electrode material. Accordingly, many efforts have been made to improve the electrical conductivity of manganese oxide-based supercapacitors. For instance, a thin manganese oxide coating disposed on the surface of a carbon material (e.g., carbon black, carbon nanotubes, or vapor grown carbon fibers) has been studied to achieve supercapacitive properties on the manganese oxide layer and to attain high-rate characteristics based on the high electrical conductivity of the carbon material.
For example, Korean Patent No. 622737 discloses an electrode for a supercapacitor that is fabricated by adding a carbon material, such as carbon black, carbon nanotubes, or vapor grown carbon fibers to a manganese solution and controlling the redox reaction between the carbon material such that a thin manganese oxide layer is formed on the surface of the carbon material.
According to another well-known method, an electrode for a supercapacitor is fabricated by mixing manganese oxide nanoparticles, a conductor, and a binder to prepare a paste, and coating the paste on a current collector. However, this method has a disadvantage in that the supercapacitive properties (e.g., specific capacitance) of the electrode deteriorate as the amounts of the conductor and the binder increase.